Bath-type vaporiser-condenser for a method of cryogenically separating a natural gas stream

ABSTRACT

A method of cryogenically separating a natural gas supply stream into a gas which contains the most volatile compounds of the supply stream, and a liquid product which contains the heaviest compounds of the supply stream, including the following steps: at least partially condensing a natural gas supply stream in a first heat exchanger system; introducing the liquid product into a fractionation column in order to obtain, in the bottom of the fractionation column, a liquid produce that contains the heaviest compounds of the supply stream and, at the top of the fractionation column, a distillate that is at least partially condensed; introducing, at a stage in the upper part of the absorption column, the gaseous phase of the condensed distillate as a supply stream for the absorption column; where the distillate is condensed in a bath-type vaporizer-condenser installed in a casing mounted on the fractionation column.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT Application PCT/FR017/053046, filed Nov. 8, 2017, which claims priority to French Patent Application No. 1660780, filed Nov. 8, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile compounds of the feed stream and into a liquid product containing the heaviest compounds of the feed stream.

During the exploitation of natural gas deposits, numerous stages may be provided. A relatively conventional stage after the drying and the withdrawal of the impurities is the separation of the liquids associated with the natural gas (NGLs).

It is often desirable to separate the heavy hydrocarbons, or more generally the NGL (Natural Gas Liquids), from the natural gas, for example such as ethane, butane, propane or C5+ and C6+(that is to say, having at least five carbon atoms and having more than six carbon atoms) hydrocarbons.

This stage can have many advantages but often it is a matter of upgrading various products (ethane, propane, and the like) which are generally sold at a much higher price than the natural gas product. It is in particular common to sell hydrocarbons having a least three carbon atoms as propane, butane and condensate products.

Many industrial installations have been described which make it possible to fractionate gas feedstocks into a residual gas containing the most volatile compounds of the feedstock and into a liquid product containing the heaviest compounds of the feedstock, this being done for the purpose of obtaining, in said liquid product, a given component of the feedstock with a high degree of recovery.

In this regard, mention may be made, for example, of the recovery of liquefied petroleum gas (hydrocarbons therein having three or four carbon atoms) from natural or refinery gas, the recovery of ethane intended in particular to feed steam cracking units, or the desulfurization and the gasoline extraction of natural gases by recovery of the sulfur-comprising compounds, such as carbon oxysulfide and mercaptains

Several technologies exist for producing hydrocarbons having at least three carbon atoms from natural gas.

One of the most effective is a process employing a two-column turbo-expander in which the first column is an absorber dedicated to forcing the recovery of as much propane as possible and the second column is a de-ethanizer.

The condensation of the de-ethanizer top stream is often carried out in part with the fluid coming from the absorber bottom.

Such a process is described in the documents U.S. Pat. Nos. 4,690,702 and 5,114,450.

Such processes can prove to be complicated to control and require investment in fairly expensive components, such as pumps.

SUMMARY OF THE INVENTION

The inventors of the present invention have thus developed a solution which makes it possible to solve the problems raised above.

A subject matter of the present invention is a process as defined in claims 1 to 4.

According to another embodiment, another subject-matter of the invention is an apparatus as defined in claim 5.

The stream of hydrocarbons to be liquefied is generally a stream of natural gas obtained from natural gas fields, oil reservoirs or a domestic gas network in which the gas is distributed via pipelines.

Generally, the natural gas stream is essentially composed of methane. Preferably, the feed stream comprises at least 80 mol % of methane. Depending on the source, the natural gas contains quantities of hydrocarbons heavier than methane, such as, for example, ethane, propane, butane and pentane and also certain aromatic hydrocarbons. The natural gas stream also contains nonhydrocarbon products, such as H₂O, N₂, CO₂, H₂S and other sulfur-comprising compounds, mercury and others.

The feed stream containing the natural gas is thus pretreated before being introduced into the heat exchanger making possible the first stage of cooling of the process which is a subject matter of the present invention. This pretreatment comprises the reduction and/or the removal of the undesirable components, such as CO₂ and H₂S, or other stages, such as the precooling and/or the pressurization.

Given that these measures are well known to a person skilled in the art, they are not described in further detail here.

The expression “natural gas” as used in the present patent application relates to any composition containing hydrocarbons, including at least methane.

This comprises a “crude” composition (prior to any treatment or scrubbing) and also any composition which has been partially, substantially or completely treated for the reduction and/or removal of one or more compounds, including, but without being limited thereto, sulfur, carbon dioxide, water, mercury and certain heavy and aromatic hydrocarbons.

The heat exchanger can be any heat exchanger, any unit or other arrangement suitable for making possible the passage of a certain number of streams, and thus making possible at least one system for direct or indirect exchange of heat between one or more liquid coolant lines and one or more feed streams.

A bath vaporizer-condenser is, for example, of the type as described in the patent application EP 1 087 194.

The invention will be described in a more detailed manner with reference to the FIGURE.

BRIEF DESCRIPTION OF THE DRAWING

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 illustrates a schematic representation of one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the FIGURE, a feed stream of natural gas 1 is introduced into a main heat exchanger 2 in order to be cooled. The gas thus cooled 3 is partially condensed and introduced into a phase separator 4. The gas phase 5 at the phase separator 4 top is introduced into a turbine 6 in order to recover the expansion energy and to condense a portion of the stream 5, and is then introduced into an absorption column 7 comprising a lower part 7′ and an upper part 7″. The liquid phase 8 at the phase separator 4 bottom is introduced after expansion 9 into the absorption column 7. The absorption column produces a liquid 10 at the column bottom and a residual gas 11 at the column top. The liquid 10 is reheated in a heat exchanger 12 in which it is partially evaporated. The stream thus reheated 13 is subsequently introduced into the main exchanger 2; this introduction 13 is thus strongly a two-phase introduction.

At the absorption column 7 top, the residual gas 11, which contains only the products more volatile than ethane, is reheated in the main heat exchanger 2; the stream which results therefrom 14 is subsequently compressed and sent to a treatment unit A.

The stream 13′ at the heat exchanger 2 outlet resulting from the bottom of the absorption column 7 is introduced into a fractionation column 15.

This column 15 produces, at the bottom 16, a reboiled liquid product 18 using a reboiler 17 in order to obtain a liquid rich in propane and depleted in ethane.

A gas 20 is produced at the fractionation column 15 top 19. This gas 20 is condensed in the heat exchanger 12 and the product 21 which exits from this exchanger 12 is introduced into a phase separator 22. The gas phase 23 at the top of the phase separator 22 acts as reflux in the absorption column 7. The liquid 25 at the bottom of the phase separator 22 acts as reflux 26 at the top of the fractionation column 15. A pump is not necessary to pump the liquid 25. This arrangement with the use of a bath vaporizer-condenser exhibits the advantage of being very easy to control while integrating the exchanger and the fractionation column and without using a pump (thermosiphon scheme).

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1.-5. (canceled)
 6. A process for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile compounds of the feed stream and into a liquid product containing the heaviest compounds of the feed stream, comprising at least the following stages: Stage a): at least partial condensation of a natural gas feed stream in a first heat-exchange system; Stage b): introduction of the at least partially condensed stream resulting from stage a) into an absorption column at an introduction level located in the lower part of said absorption column, said absorption column producing, at the top, a gas stream containing the most volatile compounds and, at the bottom, a liquid product; Stage c): introduction of the liquid product resulting from stage b) into a fractionation column in order to obtain, in the fractionation column bottom, a liquid product containing the heaviest compounds of the feed stream and, at the fractionation column top, an at least partially condensed distillate; Stage d): introduction, at a level located in the upper part of the absorption column, of the gas phase of the condensed distillate resulting from stage c) as feed stream of the absorption column; wherein the distillate resulting from stage c) is condensed in a bath vaporizer-condenser installed in a shell surrounding the fractionation column.
 7. The process as claimed in claim 6, wherein the bath vaporizer-condenser contains at least one heat exchanger fed with refrigerating fluid by the liquid product resulting from stage b).
 8. The process as claimed in claim 6, wherein the liquid phase of the condensed distillate resulting from stage c) is used as reflux at the top of the fractionation column.
 9. The process as claimed in claim 6, wherein no pump is necessary for setting up said reflux.
 10. A unit for the cryogenic separation of a natural gas feed stream utilizing the process as defined in claim 6, wherein a pipe connects the top of the fractionation column to a heat exchanger surmounting said fractionation column, which heat exchanger is itself placed in a shell provided for a bath formed predominantly of hydrocarbons. 